This study is concerned with the cellular and molecular effects of fast neutrons on cultured mammalian cells. The experiments involve the production of damage in intracellular DNA which is measured, along with an assessment of the cells' ability to repair the damage. The results with fast neutrons will be contrasted with those obtained with 250 kvp X-rays. The data will provide estimates of the RBE and OER for fast neutrons using both molecular and cellular end points. The influence of various conditions such as pH, hypoxia and available energy (ATP) on the rate and extent of DNA repair will provide information into the basic mechanisms of fast neutron effects on mammalian cells.